Breast cancer patients might have a powerful incentive to avoid gaining weight: better odds of surviving the disease.

New research suggests that for every 11 pounds a woman gains after being diagnosed with breast cancer, the chances of it proving fatal go up 14 percent.

The study is by no means definitive, but gives the strongest evidence yet that controlling weight — a good idea anytime in life — may be especially important after breast cancer.

“There was a significant trend between increasing levels of weight gain and higher mortality,” said Hazel Nichols, a doctoral student at Johns Hopkins Bloomberg School of Public Health. “Lifestyle factors, the things you incorporate after a breast cancer diagnosis such as diet and exercise, do show potential to influence survival.”

Nichols led the study and reported results Friday at an American Association for Cancer Research conference in Philadelphia.

Researchers started with 4,021 women in Wisconsin, Massachusetts and New Hampshire who had been diagnosed with breast cancer from 1988 to 2001. They gave information on their height, weight, family history and breast cancer risk factors during telephone interviews.

From 1998 to 2001, all survivors were mailed surveys asking for updated information on these factors and lifestyle habits like exercise and diet.

After an average of six years of followup since their diagnoses, 121 breast cancer deaths and 428 non-breast cancer deaths had occurred. For every 11 pounds of weight gain after diagnosis, the risk of death from breast cancer or other causes increased by 14 percent.

The link remained even after researchers took into account differences in age, menopausal status, smoking and the stage of disease when the women were diagnosed.

For women classified as obese by body mass index — a measure of weight and height — the death risk was more than twice that of women with a normal body weight.

The study was paid for by the Susan G. Komen for the Cure Breast Cancer Foundation.

“It’s a large study, it was a very well-conducted study at several centers in the United States” by well-known researchers on this topic, said Joanne Dorgan, a breast cancer scientist at Fox Chase Cancer Center in Philadelphia.

Doctors have long known that women who are overweight when they are diagnosed with breast cancer have poorer prospects.

“They’re more likely to relapse and to die of their cancer than women who are thinner,” Dorgan said.

Previous research found that women who exercised after being diagnosed with breast cancer cut their chance of dying by as much as one-half, depending on how much exercise they did.

However, it is very common for women to gain weight after being diagnosed with breast cancer. One reason may be that chemotherapy can leave them tired and ill so they don’t feel like exercising, Dorgan said.

The new work shows how important it is to get back on track and keep from gaining pounds over the long term.

“It still matters what your weight gain is after diagnosis,” said Dr. Craig Thompson, director of the Abramson Cancer Center at the University of Pennsylvania.